It is well known in the art for an integrated circuit image sensor to include an array of microlenses positioned above an array of photosensitive circuits (such as a photodiode array). A typical implementation is presented in FIG. 1 showing a cross-sectional illustration of an integrated circuit image sensor. The integrated circuit image sensor 10 includes a semiconductor substrate 12 having a top surface at which photosensitive circuits 14 are formed. Such photosensitive circuits 14 may, for example, comprise photodiode devices (such as a pinned photodiode or a single photon avalanche diode). Auxiliary electronic circuits 11, such as transistors, capacitors, etc., are also supported on and/or in the semiconductor substrate 12 and electrically coupled to the photosensitive circuits 14. An example of a MOSFET transistor comprising such auxiliary electronic circuits 11 is explicitly shown in FIG. 1. A premetallization dielectric layer 16 is provided on the semiconductor substrate 12. The premetallization dielectric layer 16 includes electrical contact structures 13 for making electrical contact to diffusions in the top surface of the semiconductor substrate 12 and to the auxiliary electronic circuits (in the illustrated example, contact structure 13 makes an electrical connection to the transistor gate and similar contact structures (not shown) may be used to connect to the source/drain regions as well as the diffusions of the photodiode). A plurality of metallization layers M1-Mn are provided above the premetallization dielectric layer 16 (where n is integer typically in the range of 3-7). Each of the metallization layers M1-Mn is formed by a layer of dielectric material 18 that includes metal wiring structures 20 and may further include metal via structures 22. A protective layer 26 is provided on the top-most metallization layer Mn. The protective layer 26 may, for example, be formed of multiple distinct layers (not explicitly shown) and include a layer configured to function as an optical filter and a layer configured to ensure a planar top surface 30. A plurality of microlenses 34 are formed on the planar top surface 30, with each microlens 34 associated with a corresponding one of the photosensitive circuits 14. The microlenses 34 are typically formed by a lithographically patterned and thermally shaped layer of organic material.
There are concerns, however, with integrated circuit image sensors implemented with microlenses 34 in the manner shown by FIG. 1. The use of organic material for the making of the microlenses 34 may be prohibited in some applications. An example of this is for image sensors used in automotive applications where the microlenses 34 may be subjected to humidity and suffer from reliability issues. Another example is in the context of 3D (three-dimensional) stacked products where the microlenses 34 cannot be processed at the passivation layer for planarity compatibility. There is a need in the art to address the foregoing problems and provide an alternative configuration for microlens structures in image sensing applications.